In semiconductor processing, a large percentage of the yield losses can be attributed to contaminations by particles and films of various nature. The contaminants may be organic or inorganic particles, films formed of polymeric bases, or other ionic based materials. The particles or films can be generated as a byproduct in the reaction of reactant gases, by the surrounding environment, by the processing equipment or by the handling of manufacturing personnel. Some contaminants are particles or films generated from condensed organic vapors, solvent residues, photoresist or metal oxide compounds.
Typical problems and the detrimental effects caused by particle or film contaminants are the poor adhesion of deposited layers, or the poor etching of the underlying material. The electrical properties and the stability of devices built on the semiconductor substrate may also be seriously affected by ionic based contaminants. The various forms of contaminants therefore not only reduce the product yield but also degrades the reliability of the devices built. For instance, contaminant particles can cause a device to fail by improperly defined patterns, by creating unpredictable surface topography, by inducing leakage current through insulating layer, or otherwise reducing the device lifetime. It is generally recognized that a particle contaminant that exceeds one-fifth to one-half of a minimum feature size on a device has the potential of causing a fatal defect, i.e. a defect that causes a device to fail completely. A defect of smaller size may also be fatal if it falls in a critical area, for instance, a particle contaminant in the gate oxide layer of a transistor. In modern high density devices, such as a dynamic random access memory chip, the maximum allowable number of particle contaminants per unit area of the device must be reduced accordingly in order to maintain an acceptable yield and reliability.
A typical low pressure chemical vapor deposition (LPCVD) system can be carried out in an apparatus as that shown in FIG. 1.
Referring initially to FIG. 1 wherein a conventional chemical vapor deposition system 10 is shown. The chemical vapor deposition method has been widely used in the deposition of silicon nitride films on semiconductor substrates. In this method, a gas containing the structural elements of the film material to be formed is fed into a chamber, followed by heating the gas mixture to induce a chemical reaction to deposit the desired film on the semiconductor substrate. In a conventional CVD method, a silicon nitride film can be deposited by a chemical reaction between silane (SiH.sub.4) and ammonia (NH.sub.3) at the atmospheric pressure and a temperature of 700.degree..about.900.degree. C., or by a mixture of dichlorosilane (SiCl.sub.2 H.sub.2) and ammonia at a reduced pressure and at a temperature of 700.degree..about.800.degree. C.
As shown in FIG. 1, reactant gases of dichlorosilane 12 and ammonia 14, each carried by a carrier gas of nitrogen, are fed into the process chamber 18 through the inner tubes 40. The reaction gases are mixed at the bottom portion of inner tubes 40. Manifold 16 provides inlets and outlets for the gases and is used as a pedestal support for the inner tubes 40 and the outer tubes 24. The process chamber 18 is first evacuated by vacuum pump 20 prior to the reaction. A purge gas of nitrogen 22 is then used to fill the process chamber 18 and to drive out any residual gas left from the previous deposition cycle. A cold trap 26 maintained at sub-ambient temperature, e.g., approximately 12.degree..about.18.degree. C. is used in the vacuum line to trap particles that cannot be pumped away. The manifold 16 is provided with a pressure sensor 28 which is connected via a conduit 30 to the manifold 16 at a pressure port 32. A main valve 34 and a sub-valve 36, as well as pressure switches 42 and 44 are provided in the vacuum evacuation line for controlling the fluid flow. A vent line 48 is connected to the vacuum evacuation line between the two pressure switches 42 and 44 for venting spent reactant gases through control valves 52 and 54 to the exhaust vent 56. The switch 42 can be a 760 Torr pressure switch, while the switch 44 can be a 800 Torr pressure switch. When the pressure in chamber 18 reaches atmospheric pressure, switch 42 sends a signal to trigger a solenoid valve to open control valve 52 so that gas can vent from the chamber. Pressure switch 44 is a backup switch which acts in case of a malfunction of switch 42. This prevents process chamber 18 from being damaged by excessive gas pressure. For instance, switch 44 sends a signal to open control valve (or air valve) 54 when chamber pressure reaches 800 Torr.
After the reactant gases of SiCl.sub.2 H.sub.2 and NH.sub.3 are mixed in the inner tube 40, the gas mixture is flown into the process chamber 18 to deposit silicon nitride films on wafers held in a wafer boat. (Not shown). It has been observed that during the reaction between SiCl.sub.2 H.sub.2 and NH.sub.3, a reaction byproduct of NH.sub.4 Cl in a white power form, is produced. The ammonium chloride powder is frequently trapped in the conduit 30 at near the pressure port 32 that is connected to the pressure sensor 28. The white powdery residue deposited at the pressure port 32 and in the conduit 30 is very difficult to remove due to the fact that there is no purge gas flowing through either the port or the conduit. The accumulation of ammonium chloride powder at the port and in the conduit creates a serious contamination problem in the process chamber 18. During a silicon nitride film deposition cycle, the ammonium chloride particles may enter into the chamber and contaminate a substrate surface that is to be deposited with silicon nitride films. The purge gas of nitrogen 22 is directly flown into the process chamber 18 at outlet 24 and also from 12 and 14 as carrier gas for the reactant gas which then exits the chamber at outlet 38. The purge gas 22, 12 and 14 therefore cannot effectively purge the area of the pressure port 32 and the conduit 30 and to carry away the contaminant particles of ammonium chloride. In order to correct this problem, the process chamber 18 has to be shut down after a predetermined number of deposition cycles such that the conduit 30 (frequently in a bellow form) may be detached from pressure port 32 and be cleaned in a wet acid process. This creates downtime in the deposition equipment and leads to reduced product yield.
It is therefore an object of the present invention to provide a method and apparatus for preventing particle contamination in a semiconductor processing chamber that does not have the drawbacks and shortcomings of prior art methods and apparatus.
It is another object of the present invention to provide a method and apparatus for preventing particle contamination in a semiconductor processing chamber that only requires a minimum modification to the existing processing equipment.
It is a further object of the present invention to provide a method and apparatus for preventing particle contamination in a semiconductor processing chamber that does not require the use of additional chemicals or gases.
It is still another object of the present invention to provide a method and apparatus for preventing particle contamination in a semiconductor processing chamber that utilizes existing purge gas to purge out contaminating particles.
It is yet another object of the present invention to provide a method and apparatus for preventing particle contamination in a semiconductor processing chamber by connecting a purge system between a conduit connecting a pressure sensing device to a manifold and the exhaust system of the processing chamber such that contaminant particles can be carried away from the processing chamber.